H istory of classification

Characteristics

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic andmulticellular,^[7] which separates them from bacteria and most protists. They are heterotrophic,^[8] generally digesting food in an internal chamber, which separates them from plants and algae.^[9] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.^[10] All animals are motile,^[11] if only at certain life stages. In most animals, embryos pass through a blastula stage,^[12] which is a characteristic exclusive to animals.

Structure

With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These includemuscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals. Typically, there is also an internal digestivechamber, with one or two openings.^[13] Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.^[14]
All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins.^[15] This may be calcified to form structures like shells, bones, and spicules.^[16] During development, it forms a relatively flexible framework^[17] upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms, like plants and fungi, have cells held in place by cell walls, and so develop by progressive growth.^[13] Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, anddesmosomes.^[18]

Reproduction and development

See also: Sexual reproduction § Animals and Asexual reproduction § Examples in animals

The use of love darts by the land snailMonachoides vicinus is a form of sexual selection

Nearly all animals undergo some form of sexual reproduction.^[19] They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova.^[20] These fuse to formzygotes, which develop into new individuals.^[21]
Many animals are also capable of asexual reproduction.^[22] This may take place through parthenogenesis, where fertile eggs are produced without mating, budding, or fragmentation.^[23]
A zygote initially develops into a hollow sphere, called a blastula,^[24] which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.^[25] In most other groups, the blastula undergoes more complicated rearrangement.^[26] It first invaginates to form a gastrulawith a digestive chamber, and two separate germ layers — an external ectoderm and an internal endoderm.^[27]In most cases, a mesoderm also develops between them.^[28] These germ layers then differentiate to form tissues and organs.^[29]